DW FORM sec offset

We should add a new form called DW_FORM_sec_offset
which is used to record offsets into an elf section.
In a non-ELF implementation it can be an offset into
any other sub-component of an executable or library.
It should be sized according to dwarf 32/64 in the same way
DW_FORM_strp is sized. (4 bytes for 32-bit dwarf. 8 bytes of 64-bit dwarf)
It's essentially a more general
form of DW_FORM_strp that can be used to replace those uses
of DW_FORM_data4/8 which record section offsets.

The dwarf standard has the abstract notion of "classes" which are
used to group forms together. The classes serve as an organizational
aid in specifying which attributes can use which FORMs.

If we create DW_FORM_sec_offset, we should also reword the class
names "lineptr" to be "section offset (lineptr)". Or possibly
"section offset (.debug_line)". This will make the list of classes
in 7.5.4 (and in Figure 3) a little more compact and easier to understand.

But 7.5.4 also says a few words about each class that explain
exactly where it points inside the relevant section. So maybe
that specificity is useful....